Resolution of chlorosilane mixtures



Aug. 22, 1950 w. PATNODE ETAL 2,519,925

RESOLUTION OF CHLOROSILANE MIXTURES Filed March 25, 1947Tetrach/orosi/ane and Trim e thy/ch ior-o si/an e Mixture Water Se arateAqueous Solution of an Alkali-Meta/ Aqueous Phase Non-Aqueous Phase H or-oxide Se ar'te Hexamethy/ Other" Methyl- Substituted Dis i/oxaneSi/oxanes Tetra (Tr/methy/sfiox 'S/Yane Inventors: Winton I. Pat'flode,John E. Eliiott,

Their Attorney.

Patented Aug. 22, 1950 RESOLUTION :OF CHLOROSILANE MIXTURES Winton I.Patnode, Richland, Wash.,-and John R.

Elliott, Schenectady, N. Y., as'signors to General Electric Company, acorporation of New York PATENT OFFICE Application Marcl125, 1947, SerialNo. 736,990

(o1. zen-448.2)

'7 Claims.

The present invention relates to a process for resolving mixtures oftetrachlorosilane (silicon tetrachloride) and trimethylchlorosilane,particularly azeotropic or constant boiling mixtures of these "twochlorosilanes. More particularly, the invention is concerned with theprocess which comprises (1) hydrolyzing a mixture of tetrachlorosilaneand trimethylchlorosilane "and (2) contacting the hydrolysis reactionproduct in an aqueous mediumwith an alkali-metal hydroxide, preferablyin the form of an aqueous solution, therebyto obtain a layer comprisinghexamethyl disiloxane.

Mixtures of the above-mentioned chlorosilanes cannot be completelyseparated into their various components by fractional distillationbecause tetrachlorosilane (B. P. 516C. at 760 mm.) andtrimethylchlorosilane .(B. P. 575 C. at 760 mm.) not only haveapproximately the same boiling points, but also form a constant boilingmixture or .azeotrope which-consists substantially of about 45'to 55 molper cent trimethylchlorosilane and'55 .to 45 mol per centtetrachlorosilane. Usually these two chlorosilanes are present inapproximately equimolecular proportions in a constant boiling mixturewhichdistills at about 545 C.at 760 mm. and contains about 65.8 per centby weightof chlorine.

Vzarious standard methods are known for resolving an azeotrope. One ofthe more common methods .is to fractionally'distill the azeotrope atpressures other than atmospheric pressure. However, with this method noappreciable change in the composition of thetetrachlorosilane-trimethylchlorosilane azeotrope is effected atpractical superor sub-atmospheric pressures. Another method involvesfractionally distilling the mixture of chlorosilanes in the presence ofa nitrile selected from the group consisting of acetonitrile andacrylonitrile as disclosed and claimed in Sauer et a1. Patent 2,388,575,issued November .6, 1945, and assigned to the same -assignee as thepresent invention. This latter method is, however, complicated by thefact that because of the corrosion of the iron equipment by theingredients employed in this method, it has .beennecessary to use an allglass still which is expensive.

In accordance with the process of our inven tion which isdiagrammatically illustrated in the accompanying drawing, we havediscovered a process for separating the components of a mix ture oftetrachlorosilane and trimethylchlorosilane thereby to obtain the lattercomponent inthe :form of hexamethyl disiloxane substan- 2 tially freeofthe former component, which process comprises first hydrolyzing themixture of chlorosilanes with an amount of Water at least in excess ofthat calculated as necessary to effect complete hydrolysis of thechlorosilanes, secondly, removing the resulting aqueous layer from thehydrolysis mixtureand contacting the substantially non-aqueous phaseresulting from the hydrolysis step with an aqueous solution, preferablya concentrated aqueous solution, of an alkalimetal "hydroxide, forexample, sodium hydroxide, potassium hydroxide, lithium hydroxide, etc.,and finally;-separat'ing the resulting substantially nonaqueous layerfromthe reaction product obtained infthe previous alkalizing step, andisolating the hexamet'hyl disiloxane present in the previously obtainednon-aqueous layer, for instance, by frac tional "distillation.

The "stepswhereby our claimed process may be carried out may be varied.The mixture of chlorosilanes may-be hydrolyzed by either adding water tothe mixture or adding the mixture to water. We have discovered that whenan azeotropeof tetrachlorosilane and trimethylchlorosilane is employedin the process, 'it is desirable that the amount of water employed inhydrolyzing the mixture of chlorosilanes be present in an amountequalto, by weight, from about to 90, preferably from to 85, per cent of thetotal weight of Water and mixture of chlorosilanes. Statedalternatively, we prefer to use, on a weight basis, from 3 to 9 partsWater per part azeotropic mixture.

"If the above-stated'proportion'of water to mixture of chlorosilanes isemployed, two layers separate-easily upon hydrolysis, one layercomprising-' -an acid aqueous phase, and the other layer comprising anon-aqueous phase consisting essentially" of a white powder which floatson the surfaceof the water. If toomuch water is employedinthe hydrolysisstep, a soupy gel is usually obtained Which is'more difficult toseparate into the two aforementioned aqueous and nonaqueous phases. Iftoo little water is employed, there is obtained a mass which isdiflicult to stir. We have found, in 'hydrolyzing the azeotrope, thatthe optimum weight ratio of water to azeotrope, on a weight basis, isapproximately rectly-with a large excess of a concentrated 'aque- Whenseparation of the hydrolysis product into 7 an aqueous phase and anon-aqueous phase has been accomplished, the former phase is removed andthe latter phase is treated with the aqueous solution of thealkali-metal hydroxide wherein the alkali-metal hydroxide is preferablypresent in a concentrated form, for example, in the form of from about a5 to normal solution. The amount of alkali-metal hydroxide employed ispreferably equal to at least that necessary to convert the silicic acidor its condensation products obtained in the hydrolysis to analkalimetal silicate, e. g., sodium silicate (NazSiOs). From theforegoing it will be apparent that the minimum amount of alkali-metalhydroxide which may be employed comprises a molar ratio of at least 2mols of the alkali-metal hydroxide per mol of silicon tetrachloride.

The aforementioned alkalizing step effects separation of the mass intoan aqueous layer and a non-aqueous or oily layer which compriseshexamethyl disiloxane and smaller amounts of other substitutedsiloxanes, for example, tetra (trimethylsiloxy) silane. Fractionaldistillation of the non-aqueous or oily layer results in good yields oftrimethylchlorosilane in the form of hexamethyl disiloxane, as well assmall amounts of trimethyl silanol (which condenses easily to formhexamethyl disiloxane) and varying amounts of condensation products oftrimethyl silanol and silicic acid, for example, the aforementionedtetra (trimethylsiloxy) silane of the general formula:

ll'CHa):

Because the reaction product after the addition of the solution of thealkali-metal hydroxide, under the proper conditions, separates soreadily into two layers, it is more practical to remove the aqueouslayer prior to isolation of the hexamethyl disiloxane. However, insteadof separating the layers, the methyl-substituted siloxanes may beremoved from the alkaline reaction mass by other methods, for example,by steam distillation or by solvent extraction with non-polar solvents,e. g., benzene, toluene, xylene, petroleum ether, diethyl ether, etc.

Hexamethyl disiloxane obtained in the practice of our claimed inventionis useful for making liquid, oily, linear hydrocarbon-substituted (e.g., methyl-substituted) polysiloxanes of the type disclosed and claimedin Patnode application Serial No. 463,814, filed October 29, 1942 (nowU. S. 2,469,888, issued May 10, 1949), and assigned to the same assigneeas the present invention. These liquid polysiloxanes, which are usefulfor lubricating purposes, generally have from about 2.001 to 2.2hydrocarbon groups per 4 silicon atom. In addition, we have found thatthe condensation products of trimethyl silanol and silicic acid are alsouseful as a source of trimethyl siloxy groups ((CHa): Si-O) forterminating the ends of hydrocarbon-substituted polysiloxane chainshaving a ratio of about 2 hydrocarbon groups per silicon atom, forexample, two methyl groups per silicon atom, by effecting rearrangementby the process of cleavage and condensation between the hydrolysisproduct of a dihydrocarbon-substituted dihalogenosilane and theaforementioned condensation product of trimethyl silanol and silicicacid (see the aforementioned Patnode application as well as Sprung eta1. application Serial No. 702,590, filed October 11, 1946 (now U. S.2,483,158, issued September 27, 1949), and assigned to the same assigneeas the instant application).

It is desirable to acidify the non-aqueous or oily layer obtained as aresult of the alkalizing step with a dilute acid, for example, a diluteaqueous hydrochloric acid solution, for instance, a 5 per cent solution,prior to fractional distillation of the mass to isolate the hexamethyldisiloxane or other substituted methyl siloxanes. This acidificationstep effects condensation of free silanol groups present in the massthereby increasing the yields and simplifies the distillation of, e. g.,hexamethyl disiloxane.

In order that those skilled in the art may better understand how thepresent invention may be practiced, the following examples are given byway of illustration and not by way of limitation.

Example About 400 grams (containing 1.44 mols SiCl4) of an azeotrope oftetrachloro'silane and trimethylchlorosilane (the trimethylchlorosilanebeing equivalent to 103 grams hexamethyl dislloxane) was added withvigorous stirring through a dropping funnel to about 2000 parts waterover a period of- 15 minutes. During the hydrolysis, the temperaturerose spontaneously to about to C. If desired, the hydrolysis may becarried out with ice water to maintain a lower temperature, or otherexternal cooling means may be employed to prevent the temperature fromrising above the boiling point of the azeotrope.

After the mixture of chlorosilanes had been added, the stirring wascontinued for about one half hour to "powder the hydrolysis product andto whip air into the slurry in order to float the powder; After allowingthe lower aqueous layer to settle completely, the latter layer wasdrained from the powder and the powder was washed twice with water.

A solution of about 500 grams (12.5 mols sodium hydroxide) sodiumhydroxide dissolved in approximately 500 grams water was then addedslowly through a dropping funnel to the washed powder, the sodiumhydroxide thus being present in a molar ratio of approximately 8.6 molsof the latter per mol of S1014, taking into account the amount of SiCliin the azeotropic mixture. The powder dissolved with the evolution ofheat and two layers separated immediately. The contents of the flaskcontaining the reaction product was drained into a separatory funnel andthe oily phase collected. Theyield of this non-aqueous or-oily phasewhich was about 117 grams, was subjected to a fractional distillationstep to yield about 70 grams of an oily product which consistedessentially "of hexamethyl disiloxane..

Tetra (trimethylsi ox ila iewas i elat dirom with-watera mixture oftetrachlorosilane and trithe residue. This commune has the followingmethylchlorosilane, (2) contacting the nonaqueproperties: ous phaseofthe hydrolysis product with an aque- B. P.=130.8-'l31;0 c. at 50 mm.Wewereefeeellelrmmhydroxide thereby N :13894 s to obtain a layer rich inhex-amethyl d1s1loxane, 01420203677 the alkali-metal hydroxide beingpresent in a culated 2.40 methyl groups per silicon atom),

37.38 per cent carbon (calculated 3.71%), and 9.32 per cent hydrogen(calculated 9.43 per cent).

An oily or non-aqueous phase obtained in accordance with the process inthe example imratio oflrom 2 to 8.6 mols ofthe latterper moltetrachlorosilane, and (3) isolating the hexamethyldisilcxane present inthe aforesaid layer.

3. The process which comprisesll) hydrolyz-ing with water an azeotropicmixture of tetrachlorosilence and trimethylchlorosilane, (2) contactingthe non aqueous phase .of the hydrolysis product with an aqueoussolution of sodium hydroxide,

mediately after the alkalizing step was found .to 1.6 thereby t0 Obtaina bstantially -aqueous have a methyl to silicon ratio .of 2.70. Varyinglayer P D-E h l ethyl disiloxane, the S0- amounts of .this oily phaseand the tetra (tridium ydroxide bein p nt n a ol r atio of methylsiloxy)silane were eachsha-ken with octa- E0111 2 t0 3- 111015 t e t 'p r molof tetramethyl tetrasiloxane (having a methyl to silicon 61112105112119,and (3) isolating e Xa et y ratio M20) in the presence .of asmall amount2c disiloXene present in e non-aqueous ye of concentrated sulfuric. acidin accordance with The Process for Separating h p n -ts the methoddisclosed in the aforementioned Pat- Of a i u f tetraehloresilene and mt y node application. The oils-obtained were Washed chlm'osilane thereby0 Obtain the latter "compo.- several times with water and centrifuged toremerit in the m of X methyl disiloxane-submove the last traces ofwater. The following stem-1am?" free of the firmer m-1 m which tableshows the properties of the oils containing P196955 flompfisis (1) yolyzing with waterthe varying proportions of the aforementioned twomlxture of chklrosfianes, ramfiving'the eq compositions intercondensedwith the octamethyl 5 ayer resulting from the hydrolysis mixture,tetrasiloxane. The number 2.7 in the first col- (3) COP-taming th ustantially n h eq -e s umn of the table refers to the number of methyl3o Port-ion resulting m e hydrolysi p in ps p r Silicon atom of thenon-aqueous with a concentrated aqueous solution of an alkalioily layerobtained as in the example after the m tal hyd x d the latter being p snt in a alkalizing step. The number 2.4 in the same molar ratio of from2 to 8.6 mols of the alkalicolumn refers to the number of methyl groupsmetal hydroxide per mol of the tetrachlorosilane, per silicon atom oftetra (trimethylsiloxy) silane. 3s nd (4) separating t resultingSubstantially The term tetramer refers o Oc amethyl tetranon-aqueouslayer from the reaction mass in (3) SIIOXaIle havlng the Structuralformula and isolating the hexamethyl disiloxane present (CH3), therein.

F0 P 5. The process for separating the components of an azeotropicmixture of tetrachlorosilane and O Shaman trimethylchlorosilane therebyto obtain the latter oHer-si component in the form of hexamethyldisiloxane substantially free of the former component, which Table eester are Type 011 Me/ S11 Chain I??? S 32 ature 00- Point smppe 150 o.100 0 210 0. efiicient 2. 0033 0. 989 11.3 1, 320 43s 0. 668 -38 2.0075 1. 037 11. 2 253 -43 Tetramer +2.4 2.0152 3. 933 12.4 3, 714 -45 2.0222 5. 736 13. 7 51. 5 20. 7 0. 594 -e3 2. 0351 9. 073 17. 7 30. s 12.5 0. 594 -67 2. 0068 1. 035 10. 7 565 230 0. 594 -3a 2. 0127 1. 939 11.5171 66. 1 0. 604 Tetramer +2.7 2.0253 3.821 13.0 57.0 23.1 0.594 -43 2.0376 5. 713 14. 9 32. 9 13. 5 0. 539 -45 2. 0605 9. 168 20.4 19. 5 s. 20. 520 -67 2. 0865 13. 070 27. e 14. 1 5. 54 0. 607 -70 1 This refers tothe ratio of methyl groups to silicon atoms in the completechain-stoppered oil. 2 This refers to the per cent, by weight, of theliquid oily phase or of tetra (trimethylsiloxy) silane intercondensedwith the octamethyl tetrasiloxane.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. The process which comprises 1) hydrolyzing with water a mixture oftetrachlorosilane and trimethyl chlorosilane, (2) contacting thehydrolysis reaction product in an aqueous medium with an alkali-metalhydroxide thereby to obtain a layer comprising hexamethyl disiloxane,the alkali-metal hydroxide being present in a ratio of from 2 to 8.6mols of the latter per mol tetrachlorosilane, and (3) isolating thehexamethyldisiloxane present in the aforesaid layer.

process comprises (1) hydrolyzing the azeotropic calculated as necessaryfor the complete hydrolysis of the two components, (2) removing theresulting aqueous layer from the hydrolysis mixture, (3) thoroughlymixing the remaining substantially non-aqueous powdery portion with aconcentrated aqueous solution of sodium hydroxide, the latter beingpresent in the molar ratio of from 2 to 8.6 mols of the sodium hydroxideper mol of the tetrachlorosilane, (4) separating the resultingsubstantially non-aqueous liquid layer from the 2. The process whichcomprises (1) hydrolyzing aqueous layer, and (5) isolating hexamethyldisiloxane from the non-aqueous layer obtained in (4).

6. The process for separating the components of an azeotropic mixtureconsisting substantially of about 45 to 55 mol per centtrimethylchlorosilane and 55 to 45 mol per cent tetrachlorosilanethereby to obtain the former component in the form of hexamethyldisiloxane, which process comprises (1) hydrolyzing the azeotropicmixture with water present, by weight, in an amount equal to from about75 to 90 per cent of the total weight of water and azeotropic mixture ofchlorosilanes, (2) separating the resulting substantially nonaqueousportion, (3) treating the latter non-aqueous portion with a concentratedaqueous solution of sodium hydroxide, the latter being present in amolar ratio of from 2 to 8.6 mols of the sodium hydroxide per mol of thetetrachlorosilane, and (4) isolating hexamethyl disiloxane from thenon-aqueous phase resulting from the alkaline treatment in (3) '7. Theprocess which comprises (1) hydrolyzing an azeotropic mixture oftetrachlorosilane and trimethylchlorosilane with water present, byweight, in an amount equal to from about 75 to 90 per cent of the totalWeight of water and azeotropic mixture of chlorosilanes, (2) separatingthe resulting substantially non-aqueous portion, (3) treating the latternon-aqueous portion with a. concentrated aqueous solution of sodium hy-REFERENCES CITED The following references are of record in the file ofthis patent:

. UNITED STATES PATENTS Number Name Date 2,211,704 Robinson Aug. 13,1940 2,381,139. Sauer Aug. '7, 1945 2,388,575 Sauer Nov. 6, 19452,389,804 McGregor Nov. 27, 1945 2,412,470 Norton Dec. 10, 19462,441,320 Hyde May 11, 1948 2,453,092 Hyde et a1. Nov. 2, 1948 OTHERREFERENCES Bauer: J our. Amer. Chem. 800.," vol. 66 (1944), pages1707-1710.

Mellor: Modern Inorganic Chemistry, 1939, Longmans, Green & 00., page693.

Robison and Kipping: Journal Chem. Soc." (London), vol. 101 (1912) pages2158, 2159.

1. THE PROCESS WHICH COMPRISES (1) HYDROLYZING WITH WATER A MIXTURE OFTETRACHLORASILANE AND TRIMETHYL CHLOROSILANE, (2) CONTACTING THEHYDROLYSIS REACTION PRODUCT IN AN AQUEOUS MEDIUM WITH AN ALKALI-METALHYDROXIDE THEREBY TO OBTAIN A LAYER COMPRISING HEXAMETHYL DISILOXANE,THE ALKALI-METAL HYDROXIDE BEING PRESENT IN A RATIO OF FROM 2 TO 8.6MOLS OF THE LATTER PER MOL TETRACHLOROSILANE, AND (3) ISOLATING THEHEXAMETHYLDISILOXANE PRESENT IN THE AFORESAID LAYER.